Orthodontic brackets may be used with arch wires in orthodontic treatment for straightening teeth. The brackets, which are attached to the surfaces of the teeth, typically include a main member portion with a generally U-shaped slot formed in the center therein for holding the arch wire. A pair of wings are formed integrally in the upper and lower portions of the bracket main member, enabling ligature means to be tied to the bracket to retain the arch wire in the slot. Tensile and bending forces from the arch wire can thereby be applied to the tooth.
During orthodontic treatment, the brackets are first attached to tooth surfaces using adhesives. The arch wire is then positioned within the slots in each bracket main member, and ligature means are tied to the wings to retain the arch wire in the slots. Stainless ligature wire or elastomeric ligature rings are conventionally used as the ligature means.
The arch wire is generally firmly tied to the brackets. However, when prescribed shifting of teeth is required during orthodontic treatment, the brackets attached to the teeth must be capable of shifting or moving along the arch wire as the teeth shift. Proper tooth shifting is difficult unless the brackets can move smoothly along the arch wire.
Moreover, it is now possible to apply weak continuous loads to the teeth using superelastic arch wires. It is, therefore, important that brackets be able to move along the arch wire without excessive friction therebetween.
When stainless steel ligature wire is used as the ligature means with prior art brackets, the arch wire is firmly tied to the bracket as the ligature wire is in contact with the underside of the wings and with the arch wire under strong pressure. The arch wire thereby becomes firmly attached to the bracket. As a result, shifting the bracket smoothly along the arch wire is difficult because of friction between the arch wire and the bracket.
If ligature wire is to be used in orthodontic treatment that requires the bracket to shift along the arch wire, it is necessary to form a gap between the ligature wire and the arch wire by positioning the ligature wire away from the arch wire. This type of ligature work is, however, difficult.
Elastomeric ligature rings may also be used as the ligature means to secure the arch wire within the slot. Use of elastomeric ligature rings is relatively simple because of the elasticity of the rings. However, even with elastomeric ligature rings, the arch wire becomes firmly attached to the bracket main member because a portion of the ligature ring will be under strong elastic pressure against the arch wire. As a result, it is difficult to shift the bracket smoothly along the arch wire.
In an attempt to solve the problems with prior art brackets, an improved bracket 10 shown in FIG. 9 was described in Japanese Patent Public Disclosure Hei 3-21236. The bracket 10 includes a bracket main member 11 having a U-shaped slot 12 formed in a central portion therein for holding an arch wire (not shown). A pair of wings 13 for retaining ligature means (not shown) are formed on the upper and lower portions of bracket main member 11. The bracket 10 differs from prior brackets in that shoulder portions 14 for supporting the ligature means are integrally formed outside the slot 12 on both sides of bracket main member 11.
With the improved bracket 10, regardless of whether stainless steel ligature wire or elastomeric ligature rings are used as the ligature means, the arch wire is tied to the bracket so that a gap is formed beneath the ligature means and the arch wire. The ligature means is supported on the upper surfaces of shoulder portions 14 on both sides of bracket main member 1, thereby enabling the gap to be formed between the ligature means and the arch wire. As a result, there is no pressured contact between the ligature means and the arch wire. Consequently, as the tooth shifts during orthodontic treatment, the bracket attached to the tooth surface can shift smoothly along the arch wire.
The shoulder portions 14 of the bracket are integrally connected to the sides of bracket main member 11, enlarging the width of the bracket 10 at the two shoulder portions 14. Small brackets are, however, preferred for orthodontic treatment. The presence of the shoulder portions 14 increases the difficulty of pushing the arch wire inside the slot 12. This problem is particularly apparent when using the brackets on the front row of teeth in the lower jaw where distances between brackets are small. Also, the process of forming the shoulder portions 14 on the bracket 10 is difficult and expensive. ,
The presence of shoulder portions 14 makes it possible to reduce pressured contact between the ligature means and the arch wire, which is desirable for cases in which the bracket 11 should shift along the arch wire. However, for orthodontic treatment requiring the bracket 10 to be firmly tied to the arch wire, the presence of the shoulder portions 14 makes firm attachment of the arch wire and bracket main member 11 difficult.